Tablet coating, which is a treatment for forming a thin film-like substance on a tablet surface, is carried out for the purpose of imparting various functions, depending on the composition to be coated, such as improved appearance, water resistance, improved friability, light stability, and distinguishability.
Orally disintegrating tablets which rapidly disintegrate in an oral cavity have now been developed as an easy-to-take formulation for patients with impaired swallowing function due to various causes such as diseases, aging, and decreased saliva production. The orally disintegrating tablets are special formulations which, in view of easiness for patients to take, are required to have an overwhelmingly high disintegration rate compared to that of general tablets and pursue high disintegration rate. However, rapid disintegration properties and high tablet hardness are generally contradictory properties, and therefore orally disintegrating tablets cause chips and cracks of the tablets when divided because of their insufficient tablet hardness and high friability, and it has been pointed out that the orally disintegrating tablets have a problem in handling at dispensing sites. Further, tablets are subject to influence of storage environments such as light, temperature, and humidity because they are stored in the unpacked form in a divider before being automatically divided and are stored in the simply packed form after being divided, and there has been a disadvantage that drugs sensitive to these external environments are inapplicable. In this regard, for example, in cases where a light-unstable drug is tableted, the tablet is generally stabilized by being coated with a coating layer to which a light shading agent is added, but in the case of the orally disintegrating tablets, there is a concern that disintegration is delayed by the coating layer, and stabilization by light shading coating on the tablet itself is difficult.
In addition, many of the orally disintegrating tablets cause a volume expansion under high humidity because the orally disintegrating tablets are provided with rapid disintegration properties by the addition of a hygroscopic excipient and a powerful disintegrator. Therefore, in order for an orally disintegrating tablet to be a coated tablet, it requires not only rapid disintegration properties but also extensibility of the coating layer, but at present there are no conventionally known coated compositions which have an ability to withstand the expansion of a tablet while maintaining orally disintegrating properties.
On the other hand, it is conventionally known that polyvinyl alcohol is used as a coating polymer in the general tablets. There are disclosures of preventing crack occurrence in a coating layer during storage by coating an expansible tablet with polyvinyl alcohol (for example, Patent Document 1). There are also disclosures of preventing crack occurrence in a coating layer during storage by coating an expansible tablet with polyvinyl alcohol with saccharides and a viscosity reducer being added (for example, Patent Document 2). Further, there are disclosures of reducing cracks of the tablet by providing a coating layer of water-soluble polymer and saccharides as an intermediate layer of a sugar-coated tablet (for example, Patent Document 3). However, all these reports only disclose that polyvinyl alcohol films exhibit excellent extension and do not disclose that when polyvinyl alcohol is coated on an orally disintegrating tablet, the tablet exhibits such an excellent rapid solubility that allows easy taking in an oral cavity.
On the other hand, another example of the easy-to-take formulation for the above-described purpose is an orally disintegrating film preparation which disintegrates in an oral cavity. As the film preparation, compositions obtained by adding saccharides to water-soluble polymer such as hydroxypropylmethylcellulose are disclosed (for example, Patent Document 4), but these disclosures mainly focus on disintegration properties and do not describe film extension. Rather, film extension in the film preparation, for example, causes undesirable forms such as a softened film, and is essentially different from the requirements for the coating layer of an orally disintegrating tablet.
Further, there are disclosures of methods of forming a coating layer by melting soluble powder such as low melting point polymer in order to improve the insufficient tablet hardness and high friability of an orally disintegrating tablet (for example, Patent Document 5). However, melting technique, by which it is difficult to form a coating layer uniformly on the whole tablet, is inadequate as a light stabilization method. As a dosing preparation for a drug absorbed via oral mucosa, preparations obtained by coating an orally disintegratable core with a composition composed of gellan gum are disclosed (for example, Patent Document 6). However, the extension of the coating layer is not reported at all.
On the other hand, as a method of stabilizing nalfurafine hydrochloride, there are disclosures of solid preparations containing sodium thiosulfate, saccharides or sugar alcohols, and low-substituted hydroxypropylcellulose, the disclosures also describing that the preparations are coated tablets (for example, Patent Document 7 and Patent Document 8). However, they only disclose that general coated compositions can be coated and do not describe at all the excellent rapid disintegration properties necessary for orally disintegrating tablets or the effect of preventing cracks in a coating layer during storage, and therefore the present invention cannot easily be inferred from these reports.